A conventional synchronous motor typically uses rotor position sensors to provide information regarding the position of the motor's rotor with respect to the motor's stator windings. Rotor position sensors, such as Hall effect devices, are typically mounted in the stator, proximate to the stator windings. The rotor position sensors provide rotor position information which allows for the proper control for the conversion of power that is supplied to the stator windings of an electrical machine.
However, rotor position sensors can be unreliable due to mechanical alignment problems (e.g., problems caused by bearings) and temperature incompatibility problems between the stator windings and the electronic components, such as the incorporated Hall effect devices. Moreover, the rotor position sensors can be difficult to mount to the motor during motor assembly, especially for multi-pole motors. In such multi-pole motor assemblies, the electrical misalignment angle is equivalent to the angular mechanical misalignment angle multiplied by the number of pairs of poles. In response to these and other problems with rotor sensors, several sensorless position control techniques have been developed for controlling the speed of the synchronous machines.
For example, U.S. Pat. No. 6,301,136 to Huggett et al., which is incorporated herein by reference in its entirety, discloses a floating reference frame controller which advantageously provides a floating synchronous reference frame for controlling an inverter to drive a synchronous motor. FIG. 1 is a high level block diagram which is provided to illustrate such a controller model in a floating frame controller. Specifically, FIG. 1 illustrates a synchronous machine drive system 10 including a three-phase synchronous machine 12, and an inverter 14 to be coupled to a power source for supplying dc power to the inverter 14. During operation of the synchronous machine 12, the inverter 14 converts the dc power to three-phase ac power and supplies currents to all three stator windings of the machine 12. These currents flow through the windings and create a rotating magnetic field.
The system 10 estimates the reference frame without using position sensors, and further includes a set of current sensors 20 for sensing current on the power line and a floating frame controller 22 for controlling the inverter 14 to convert the dc power to suitable three-phase ac power. Each current sensor 20 is synchronously and periodically sampled. Thus, a set of current samples (i.sub.a, i.sub.b, i.sub.c) is periodically produced.
However, due to the coupling impact of the motor circuitry in the floating synchronous reference frame, delays exist during transients, during which, the estimated angle of the current Park vector differs from the actual angle of the current Park vector. Also, the coupling impact of the motor equivalent circuit in the floating synchronous reference frame forces the gain of the regulator used for angle estimation to be kept small, which slows the system response.
A further result of using a small gain is a significant delay in reducing the error between the estimated angle of the current Park vector and the actual angle of the current Park vector during slow transients, such as acceleration and deceleration, as well as fast transients, such as step changes. Because undesirable imaginary axis current flows in the machine during these delays, longer delays may require substantial overrating of the inverter and electrical machine. Ideally, only real-axis current in floating reference frame flows in the machine, as the floating synchronous reference frame is aligned with the real axis component of the current Park vector.
In prior schemes for sensorless control, the current vector is either in-phase with the terminal voltage vector as illustrated by the disclosure of U.S. Pat. No. 6,301,136 referenced above, or not in-phase in an uncontrolled fashion due to the application of the decoupling of crosscoupling concept as illustrated by the disclosure of U.S. patent application Ser. No. 10/834,857 referenced above. However, in some applications, it is desirable to control or actively change the power factor during the operation to achieve desired system characteristics and requirements.
In yet another scheme for sensorless control, the power factor control is achieved using both ac and dc current sensors as illustrated by the disclosure of U.S. patent application Ser. No. 11/174,550 referenced above. In this method, at least two motor phase currents are measured and a floating reference frame for the current Park vector is obtained. The reference frame is adjusted based on an estimated rotor speed, and the power converter is controlled via the floating reference frame. However, this method requires both ac and dc current sensors.
Accordingly, there is a need to achieve power factor control providing unity, leading or lagging results, for the sensorless control of synchronous machines, whereby the cost, size, and volume can be minimized and reliability can be increased.